PROJECT SUMMARY: For amputees to exploit the full capability of state-of-the-art prosthetic limbs with rapid fine-movement control and high- resolution sensory percepts, a nerve-interface with a large number of reliable and independent channels of motor and sensory information is needed. The strongest signal sources in nerves are the nodes of Ranvier, which are essentially distributed randomly within a small 3-D volume. Thus, to comprehensively engage with the electrical activity of a nerve, a neural interface should interrogate a nerve in a 3-D volume of the same scale. To date, the clinical translatability, performance, and/or operational lifetime of all existing nerve-interfaces are either: limited to low channel counts and/or non-3-D electrode arrangements, capable of detecting single-unit activity at only very low signal amplitudes that are often swamped by noise, and/or trigger a foreign body response linked to diminished channel performance over time. Our paradigm-shifting approach for 3-D scalable nerve interfaces is to integrate a stack of multi-electrode thin-film polyimide- metal electrode arrays (?threads?) into tissue-engineered biodegradable extracellular-matrix-based hydrogel nerve scaffold. We call this new class of neural interface Tissue-Engineered Electronic Nerve Interfaces (TEENI). In preliminary studies we demonstrated that we can (1) microfabricate multi-electrode arrays that can survive high- temperature reactive-accelerated aging (RAA) soak tests through the use of amorphous silicon-carbide and titanium adhesion layers between the metal and polyimide layers, (2) form a 3-D array of electrodes by integrating a stack of polymer-metal multi-electrode arrays into an extracellular-matrix-based hydrogel scaffold wrapped with small-intestinal submucosa (SIS) to support the hydrogel, provide suturable ends for attachment to the nerve, and facilitate easy surgical handling and implantation without limiting the design of the electrode array or damaging it, (3) achieve robust regeneration of vasculature and neural fibers into the TEENI scaffold, and (4) obtain chronic recordings of single-unit activity inside TEENI implants. However, we made two observations that motivated the specific aims for this proposal. First, we observed a tight tissue response around each thread that that could limit the density of 3-D TEENI multi- electrode-thread integration. Second, we observed that only a fraction of the regenerated nerve tissue preferentially grew along the microfabricated multi-electrode arrays, with the remainder growing along the inner surface of the SIS wrap and with incompletely degraded hydrogel between the two. In Specific Aim 1 we propose to reduce the size of the foreign body response in the same manner it has been achieved with microfabricated probes implanted into brain tissue: reduce the width and thickness of the implant to ~10 m and ~1 m respectively. In Specific Aim 2 we propose to use microchannel-templated hydrogels to increase the number and uniformity of axons and Schwann cells regenerating near the TEENI microfabricated multi-electrode arrays. To gain unique insight into the performance of TEENIs, we will visualize the 3-D distribution of electrodes, nodes, axons, vasculature, and any tissue response around the interface inside the regenerated nerve by employing device-capture histology, CLARITY, and light sheet microscopy.